Effect of ionic surfactants on the phase behavior and structure of sucrose ester/water/oil systems

The phase behavior and structure of sucrose ester/water/oil systems in the presence of long-chain cosurfactant (monolaurin) and small amounts of ionic surfactants was investigated by phase study and small angle X-ray scattering. In a water/sucrose ester/monolaurin/decane system at 27 degrees C, instead of a three-phase microemulsion, lamellar liquid crystals are formed in the dilute region. Unlike other systems in the presence of alcohol as cosurfactant, the HLB composition does not change with dilution, since monolaurin adsorbs almost completely in the interface. The addition of small amounts of ionic surfactant, regardless of the counterion, increases the solubilization of water in W/O microemulsions. The solubilization on oil in O/W microemulsions is not much affected, but structuring is induced and a viscous isotropic phase is formed. At high ionic surfactant concentrations, the single-phase microemulsion disappears and liquid crystals are favored.     

Synthesis of Ag and AgI quantum dots in AOT-stabilized water-in-CO2 microemulsions

Silver and silver iodide nanocrystals have been synthesized in the water-in-CO(2) reverse microemulsions formed by the commonly used surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT), in the presence of 2,2,3,3,4,4,5,5-octafluoro-1-pentanol as cosurfactant. The nanometer-sized aqueous domains in the microemulsion cores not only act as nanoreactors, but the surfactant interfacial monolayer also helps the stabilization of the metal and semiconductor nanoparticles. The transmission electron microscopy results show that silver and silver iodide nanocrystals with average diameters of 6.0 nm (standard deviation, SD=1.3 nm) and 5.7 nm (SD=1.4 nm), respectively, were formed. The results indicate that the method can be utilized as a general and economically viable approach for the synthesis of metal and semiconductor quantum dots in environmentally benign supercritical carbon dioxide.     

Formation of zinc sulfide nanorods and nanoparticles in ternary W/O microemulsions

A simple, easy approach to the synthesis of semiconductor ZnS nanorods and nanoparticles exhibiting versatile morphology-formation ability is reported. Water-insoluble zinc sulfide nanocrystals were synthesized in ternary water-in-oil (w/o) microemulsion systems stabilized by either nonionic or, in contrast, cationic surfactant. Products were visualized by transmission electron microscopy (TEM) and identified by energy-dispersive X-ray spectroscopy (EDAX); electron diffraction (ED) was also performed for individual nanorods. With varying molar ratios of water to surfactant (omega0) in solution, hence changing droplet sizes of water pool of microemulsions consequently, several morphologies with different size spans were encountered in the formation of ZnS, such as nanorods and spherical or ellipsoidal particles. Meanwhile, product morphology was also found to be sensitive to the absolute reactant concentration and concentration ratio of [Zn2+] to [S2-], the incubation time, and the ambient temperature. A schematic mechanism for the formation of ZnS nanocrystals and their morphological diversity is described. It is feasible to extend this method to the synthesis of one-dimensional nanocrystals of other semiconductors, given suitable formulae of microemulsions and other appropriate reaction conditions.     

The structure of O/W microemulsions formulated with alkylglucosides

By following a method proposed by Kahlweit, an equilibrium diagram was determined in O/W quaternary microemulsions. These systems were formed by octanol (cosurfactant)/water/octane (oil)/alkylglucoside (surfactant). The experiment study was performed at two different temperatures (25°C and 50°C). The objective of this study work was to determine the structure of these lower microemulsions and to study the influence of the cosurfactant. Hence, different experimental techniques were employed: light scattering (static and dynamic), Kerr effect (static and dynamic), viscosity and refractometric measurements. It was concluded that the surfactant volume fraction in equilibrium with micelles is 0.044 and the micelle shapes are revolution ellipsoids.     

混合表面活性剂微乳状液的形成和相行为研究进展

讨论了单一表面活性剂,混合表面活性剂,助溶剂等对油／水微乳状液的形成和相行为的影响。对混合表面活性剂微乳状液的形成和相行为研究工作进行了归纳和总结,重点分析了正负离子表面活性剂微乳状液的相行为和表面活性剂微乳状液的相行为和表面活性剂效率,讨论了微乳状液形成的影响因素,并提出了这一研究领域可能的发展前景。     

Formation of zinc sulfide and hydroxylapatite nanoparticles in polyelectrolyte-modified microemulsions

The paper is focused on the formation of nanoparticles, i.e., zinc sulfide (ZnS) and hydroxylapatite, in a microemulsion template phase consisting of heptanol, water, and a surfactant with a sulfobetaine head group in the absence and presence of an added polyelectrolyte. In the absence of a polyelectrolyte, beside larger particles, spherical ZnS nanoparticles with a diameter below 10 nm can be redispersed after solvent evaporation. In the presence of the synthetic cationic polyelectrolyte poly(diallyldimethylammonium chloride), a reloading of the particle surface is observed, and cationic charged ZnS nanoparticles, of about 5 nm in size, can be redispersed as a main fraction. When hydroxylapatite is formed in the presence of the more stiff biopolymer chitosan hydroxylapatite, hybrid structures were formed. Transmission electron micrographs show fiber-like aggregate structures, consisting of individual small nanoparticles ordered along the polymer chain.     

BEHAVIOUR OF OIL/WATER MICROEMULSIONS UPON DILUTION WITH WATER

The dilution of series of oil-in-water microemulsions, formulated with Aerosol-OT as surfactant, water, isopropyl myristate as oil and varying amounts of butanol as cosurfactant, was studied. An infinite dilution could be obtained only adding water containing surfactant; the minimum amount of Aerosol-OT required was determined. The behaviour of the microemulsions upon dilution was related to the presence of nixed micelles in the aqueous phase.     

MICROEMULSION SYSTEMS WITH A NONIONIC COSURFACTANT

The conditions to obtain W/0 microemulsions using ionic surfactants and a nonionic cosurfactant, a polyoxyethylene alkyl ether, were investigated. The length of the polyoxyethylene chain was critical to obtain the typical water solubilization maximum

The variation of the W/0 microemulsion region with hydrocarbon content was different from that of the usual type of microemulsions having a medium chain length alcohol as cosurfactant. In the present systems the W/0 microemulsion region was not a direct continuation of the inverse micellar area at zero content of hydrocarbon. Addition of hydrocarbon was necessary for the formation of inverse micelles

The microemulsion regions were sensitive to the kind of hydrocarbon used; a sign of the importance of the nonionic surfactant for the stability of this kind of microemulsions.     

Growth of silica nanoparticles in methylmethacrylate-based water-in-oil microemulsions

The mechanism of silica particle formation in monomer microemulsions is studied using dynamic light scattering (DLS), atomic force microscopy, small-angle X-ray scattering (SAXS), and conductivity measurements. The hydrolysis of tetraethylorthosilicate (TEOS) in methylmethacrylate (MMA) microemulsions (MMA = methylmethacrylate) is compared with the formation of SiO₂ particles in heptane microemulsions. Stable microemulsions without cosurfactant were found for MMA, the nonionic surfactant Marlophen NP10, and aqueous ammonia (0.75 wt%). In the one-phase region of the ternary phase diagram, the water/surfactant ratio (R _w) could be varied from 6 to 18. The DLS and SAXS measurements show that reverse micelles form in these water-in-oil (w/o) microemulsions. The minimum water-to-surfactant molar ratio required for micelle formation was determined. Particle formation is achieved from the base-catalyzed hydrolysis of TEOS. According to atomic force microscopy measurements of particles isolated from the emulsion, the particle size can be effectively tailored in between 20 and 60 nm by varying R _w from 2–6 in heptane w/o microemulsions. For MMA-based microemulsions, the particle diameter ranges from 25 to 50 nm, but the polydispersity is higher. Tailoring of the particle size is not achieved with R _w, but adjusting the particle growth period produces particles between 10 and 70 nm.     

Formation of silver bromide precipitate of nanoparticles in a single microemulsion utilizing the surfactant counterion

Silver bromide precipitate of nanoparticles was prepared by addition of silver nitrate aqueous solution to a single microemulsion system consisting of dioctyldimethylammonium bromide, n-decanol, and water in isooctane. The silver ion reacted readily with the surfactant counterion, bromide, to form the precipitate of nanoparticles, which was stabilized in the water pools. The use of the surfactant counterion as a reactant is a new approach to nanoparticle preparation in microemulsions. It is characterized by high reactivity and less dependency on the intermicellar exchange of solubilizate. The effects of the surfactant and the cosurfactant concentrations, the amount of silver nitrate, and the water to surfactant mole ratio, R, were evaluated. Increasing the surfactant concentration at fixed R and amount of silver nitrate enhanced the role of intermicellar nucleation and resulted in the formation of larger particles, while increasing the amount of silver nitrate at fixed values of all the other variables enhanced the direct nucleation and resulted in the formation of smaller particles. Particle aggregation and flocculation took place when the concentration of n-decanol or the value of R was increased. Particle aggregation and flocculation were attributed to the decrease in the interaction between the surfactant protective layer and the nanoparticles in the water pools.     

Liquid interface functionalized by an ion extractant: the case of Winsor III microemulsions

The present work shows for the first time that tributylphosphate (TBP), the major ion extractant used in the reprocessing of spent nuclear fuel, acts efficiently as a cosurfactant in the formation of three-phase microemulsions. The system is composed of water, dodecane, TBP, and an extremely hydrophilic sugar surfactant, n-octyl-β-glucoside. The investigation of the three-phase region (Winsor III), the so-called "fish-cut" diagrams, revealed that TBP exhibits cosurfactant behavior comparable to that of classical cosurfactants n-pentanol and n-hexanol. Upon increasing the cosurfactant/surfactant molar ratio, TBP appears to be more efficient than single-chain alcohols in raising the spontaneous curvature of the adsorbed surfactant film toward oil. This is a direct consequence of the different lateral packing of TBP and n-pentanol or n-hexanol in the mixed surfactant film, with TBP having three alkyl chains and so a higher hydrophobic volume than those n-alcohols. This property is underlined by the interfacial film composition, which is determined by the chemical analysis of the excess phases. It gives a surfactant to cosurfactant molar ratio of 1:1 for TBP and 1:3 for n-hexanol. Moreover, the local microstructure of the microemulsion becomes dependent on the addition of salt when n-alcohol is replaced by TBP. A specific salt effect is also observed and rationalized in terms of the complexing property of TBP and Hofmeister's effects. Treatment of the small-angle neutron scattering (SANS) data gives access to (i) the length scales characterizing the microemulsions (i.e., the persistence length, ξ, and aqueous or organic domain sizes, D*) and (ii) the specific surface, Σ. It results that a subtle change is highlighted in the TBP microemulsion structure, in terms of connectivity, according to the type of salt added.     

Microemulsions with polymerizable surfactants. γ-ray induced copolymerization of styrene and 11-(acryloyloxy)undecyl(trimethyl)ammonium bromide in three-component cationic microemulsion

Styrene and the cationic surfactant 11-(acryloyloxy)undecyl(trimethyl)ammonium bromide (AUTMAB) form transparent, globular microemulsions in water without any addition of a cosurfactant. Upon γ-irradiation, the two monomers copolymerize at room temperature and form nanolatex particles of about 21 nm in diameter consisting of a copolymer with a mole ratio of styrene to surfactant of approximately 1.5:1. Excess surfactant is polymerized in the micellar solution by formation of homopolymer. A structure model is presented taking into account that the surfactant most likely forms the particle shell. Due to the polar, covalently bonded shell the polymer particles can be easily redispersed.     

W/O microemulsions of styrene monomer and dimer

The reason for the lower water solubility in W/O microemulsions of the styrene dimer, trans, trans-1,4-diphenyl-1,3-butadiene, in comparison with the monomer was investigated by determining the phase equilibria in the microemulsion system with pentanol as the cosurfactant and sodium dodecyl sulfate as the surfactant. The main factor for the low tolerance for water in the system was the incompatibility of the dimer with the cosurfactant and surfactant.     

Interfacial composition, thermodynamic properties, and structural parameters of water-in-oil microemulsions stabilized by 1-pentanol and mixed surfactants

The present study is focused on the evaluation of the interfacial composition, thermodynamic properties, and structural parameters of water-in-oil mixed surfactant microemulsions [(cetylpyridinium chloride, CPC+polyoxyethylene (20) cetyl ether, Brij-58 or polyoxyethylene (20) stearyl ether, Brij-78)/1-pentanol/n-heptane, or n-decane] under various physicochemical environments by the Schulman method of cosurfactant titration of the oil/water interface. The estimation of the number of moles of 1-pentanol at the interface (n(a)(i)) and bulk oil (n(a)(o)) and its distribution between these two domains at the threshold level of stability have been emphasized. The thermodynamics of transfer of 1-pentanol from the continuous oil phase to the interface have been evaluated. n(a)(i),n(a)(i), standard Gibbs free energy (ΔG(t)(0)), standard enthalpy (ΔH(t)(0)), and standard entropy (ΔG(t)(0)) of transfer process have been found to be dependent on the molar ratio of water to surfactant (ω), type of nonionic surfactant and its content (X(Brij-58 or Brij-78)), oil and temperature. A correlation between (ΔH(t)(0)) and (ΔS(t)(0)) is examined at different experimental temperatures. Bulk surfactant composition dependent temperature insensitive microemulsions have been reported. Associated structural parameters, such as droplet dimensions and aggregation number of surfactant and cosurfactant at the droplet interface have been evaluated using a mathematical model after suitable modifications for mixed surfactant systems. In light of these parameters, the prospect of using these microemulsion systems for the synthesis of nanoparticles and the modulation of enzyme activity has been discussed. Correlations of the results in terms of the evaluated physicochemical parameters have been attempted.     

The origin of thermodynamic stability of microemulsions

The adsorption of surfactant and cosurfactant on the surface of the globules decreases the interfacial tension between oil and water to very low values. In addition, the decrease of the bulk concentrations of the surfactant and cosurfactant decreases their chemical potential both in the bulk and at the interface, thus decreasing the free energy of the system (dilution effect). The thermodynamic stability of microemulsions is due to the fact that the total free energy change caused by these effects can become negative. The theory can explain the occurence of stable microemulsions for both non-ionic and ionic surfactants.     

A Visible Spectroscopic Study of Microstructure of O/W Microemulsions of Anionic Surfactants: Effect of Cosurfactant

The microstructure of o/w microemulsions, stabilized by sodium dodecyl benzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS) with different cosurfactants, has been studied by partitioning of a dye, phenol red, between the oil‐water interface and bulk water. The cosurfactants used are propan‐1‐ol, propan‐2‐ol, butan‐1‐ol, butan‐2‐ol, pentane‐1‐ol, pentane‐2‐ol, and pentan‐3‐ol. The effects of changing the oil volume fraction and surfactant‐cosurfactant w/w ratio on the oil‐water interface and droplet size have also been discussed. Larger droplet size was predicted for SDS than SDBS. The predicted droplet radius increased with increase in the oil fraction, decrease in the surfactant concentration, increase in the C‐number of the linear cosurfactant, and decrease in branching of the cosurfactant. Surfactant‐cosurfactant ratio and pH did not affect the droplet size significantly. The minimum concentrations of surfactants with which microemulsions were formed were found to be higher for larger oil fraction, smaller C‐number of the alcohol, more branching of the alcohol, and higher pH.     

Microstructure and structural transition in microemulsions stabilized by aldonamide-type surfactants

Significant efforts were undertaken to characterize the microstructure and structural properties of water-in-oil (w/o), oil-in-water (o/w), and bicontinuous (bc) microemulsions composed of N-alkyl-N-methylgluconamides (n-alkyl = n-C(12)H(25), n-C(14)H(29), n-C(16)H(33)) and n-alcohols (ethanol, n-propanol, n-butanol) or iso-alcohols (iso-propanol, iso-butanol) as cosurfactants, as well as iso-octane and water. The internal structure of so created four-component system was elucidated by means of an analysis of isotropic area magnitudes in phase diagrams and conductivity measurements. Dynamic light scattering (DLS) measurements provided the microemulsion size and polydispersity. Polarity and viscosity of microemulsion microenvironment were acquired by means of electron paramagnetic resonance (EPR), UV-vis absorption spectroscopy (in the case of w/o droplets), and steady-state fluorescence (SSF) (in the case of o/w droplets). The results show that both the surfactant and the cosurfactant types affect the shape and extent of microemulsions. The size of droplets depends strongly on the type of examined microemulsion and the type of cosurfactant (linear or brunched) but is almost independent of the length of the surfactant alkyl chain. The size of microemulsion droplets ranges from 8.1 to 22.6 nm and from 3.7 to 14.3 nm respectively, for o/w and o/w microemulsions, making them good candidates for both template-based reactions and household components solubilizing media.     

Effects of surfactant/water ratio and dye amount on the fluorescent silica nanoparticles

Effects of surfactant/water volume ratios and dye amounts on the properties of micelles and fluorescence silica nanoparticles were studied in microemulsions containing nonionic surfactant Triton X-100, hexanol as co-surfactant, cyclohexane as organic solvent, and metal organic dye (tris(2,2′-bipyridyl)dichlororuthenium) via fluorescence probe technique, TEM, and XPS. Fluorescence probe measurements show that the micelle microenvironment becomes stable at the surfactant/water volume ratio higher than 3.5 and the incubation time longer than 10 h. The data suggest that the silica shell, which is formed at the surfactant/water ratio of 3.5, yields an efficient protection of dye molecules against the e-beam irradiation and result in high photostability of fluorescent silica. We pioneered the localization of dye molecules on the surface of dye-doped silica and found that an increase of dye amounts, beyond a threshold, in the microemulsion cannot enhance the fluorescence intensity of dye-doped nanoparticles. These results are of significant importance for optimizing the synthesis of fluorescent nanoparticles with high photostability and low cost.     

Microemulsification of triglyceride sebum and the role of interfacial structure on bicontinuous phase behavior

A unique triblock surfactant is reported that allows for the efficient microemulsification of triglycerides. Unlike the results of all previous efforts, these triglyceride microemulsions can be formed without the use of cosurfactants or dilution with co-oils and follow the classical patterns of surfactant phase behavior exhibited by mixtures of water, alkane oils, and nonionic oligoethylene glycol surfactants, i.e., progression from oil/water emulsions to one-phase microemulsions to water/oil emulsions with increasing temperature. Lamellar phases that usually dominate the aqueous phase behavior of surfactant/triglyceride mixtures are suppressed, allowing for the formation of single-phase microemulsions containing equal amounts of triglyceride and water. These isotropic and low-viscous fluids are particularly useful for cleansing and delivery of functional ingredients in skin care products. The effects of mixing a variety of typical skin care ingredients and components of sebum (skin oil) were also explored. Fatty acids significantly reduce the average microemulsion temperature, while other ingredients and oils, which do not partition at the oil/water interface, have less impact on the phase behavior. In all cases, one-phase microemulsions containing equal amounts of oil and water can be formed even at high additive concentrations. Indeed, partial replacement oftriglyceride with any of the additives examined consistently reduced the amount of surfactant necessary to form single-phase microemulsions. However, the greatest boost in surfactant efficiency was found with the addition of medium molecular weight amphiphilic block copolymers.     

Thermostable biocatalytic films of enzymes and polylysine on electrodes and nanoparticles in microemulsions

Microemulsions of oil, water and surfactant were evaluated as media for biocatalysis at high temperatures employing films of polylysine (PLL) and the enzymes horseradish peroxidase (HRP), soybean peroxidase (SBP) and the protein myoglobin (Mb). PLL was covalently linked to oxidized pyrolytic graphite electrodes or carboxylated 500 nm diameter silica nanoparticles, then cross-linked by amidization to HRP, SBP and Mb. The resulting film systems were stable at 90 degrees C for >12 h in microemulsions. Characterization of the microemulsions by conductivity, viscosity and probe diffusion coefficients suggested that these media have bicontinuous microstructures from 25 to 90 degrees C. UV circular dichroism and visible spectroscopy confirmed that the enzymes retained near-native conformation in the films at temperatures as high as 90 degrees C. Oxidation of o-methoxyphenol to 3,3'-dimethoxy-4,4'-biphenoquinone by enzyme-PLL films on silica nanoparticles gave yields 3-5-fold larger in microemulsions at 90 degrees C compared to the same reaction at 25 degrees C. The best yields were in CTAB microemulsions and were 3-fold larger than in buffers at 90 degrees C.